FIG. 1 is a block diagram of a conventional two wire low voltage dimming system 10. Dimming system 10 comprises a two wire dimming circuit 24 with a pair of wires 26, 16 connected in series with the primary 18 of a transformer 21 and an alternating current (AC) supply voltage 12. Dimming circuit 24 comprises a Triac 22 having a control circuit 28 operatively coupled across the circuit 24 for supplying control signals to the gate terminal 23 of the Triac 22 for selectively rendering the Triac 22 conductive. The timing of the control signals and hence the firing angle of the Triac 22 governs the root mean square (RMS) value of the AC voltage applied to the load. The dimmer circuit 24 illustrated in FIG. 1 is shown as controlling the low voltage applied to a lamp 14 connected across secondary 20.
The firing angle of Triac 22 is governed by the instantaneous voltage across the control circuit 28, and hence across wires 26, 16. Thus, the firing angle may be affected by the phase shift caused by magnetic load, which may cause an asymmetric firing at different half cycles of the AC voltage, resulting in a direct current (DC) component that flows through the primary 18 of transformer 21. The magnitude of this DC current may become significant and cause saturation of the magnetic material in the core of the transformers and a significant increase in current capable of damaging both the transformer and the dimmer. The asymmetric firing of the Triac is due to an increased phase shift between voltage and current, which may create a condition in which the Triac cannot begin to conduct and latch its state. This is especially a problem in dimmers using short gate pulses to fire a Triac, because a 2-wire power supply cannot provide sufficient current to keep the gate of the Triac at the correct level throughout the required Triac conduction time. A large phase shift may occur, for example, if one or more of the lamps 14 burns out.
What is needed is a magnetic low voltage dimmer that reduces the DC magnetizing current that may be present in a dimmer system, and thus protects the load and dimmer from damage.